Acrylic acid is the base material of a variety of acrylates widely used for highly-absorbent resins and paints, adhesives, polishes, electrical insulating materials and processing of leather, fiber, paper, etc. With the development in polymer processing techniques, demand on acrylic acid is increasing consistently.
Acrylic acid or methacrylic acid is industrially manufactured by partially oxidizing propylene or isobutylene and then oxidizing acrolein or methacrolein, which are obtained from the partial oxidation.
That is to say, the process of manufacturing acrylic acid or methacrylic acid from partial oxidation of propylene or isobutylene comprises two reaction steps. In the first step, propylene or isobutylene is oxidized to acrolein or methacrolein using an oxide catalyst comprising molybdenum and bismuth as key constituent. In the second step, acrolein or methacrolein is oxidized again to acrylic acid or methacrylic acid using an oxide catalyst comprising molybdenum and vanadium.
In the manufacture of acrylic acid or methacrylic acid, selectivity improves with the performance of the catalyst. Hence, researches for improving performance of the Mo—Bi catalyst are being performed actively.
U.S. Pat. No. 2,941,007 (J. L. Callahan, et al.) disclosed a catalyst comprising bismuth molybdate or bismuth phosphomolybdate. U.S. Pat. No. 3,171,859 (K. Sennewald, et al.) disclosed a catalyst comprising Fe, Bi, P, Mo and O. U.S. Pat. No. 3,522,299 (S. Takenaka, et al.) disclosed a catalyst comprising Ni, Co, Fe, Bi, Mo, P, As and O. U.S. Pat. No. 3,089,909 (J. L. Barclay, et al.) disclosed a catalyst comprising a compound selected from the group consisting of tin tungstate, tungstic acid and bismuth tungstate. U.S. Pat. No. 3,825,600 (T. Ohara, et al.) disclosed a catalyst comprising Mo, Co, Fe, Bi, W, Si and an alkali metal. Korean Laid-Open Patent Publication No. 2003-18917, Korean Laid-Open Patent Publication No. 2002-43801, U.S. Pat. No. 4,873,217 and U.S. Pat. No. 4,224,187 disclosed methods of improving catalytic performance by a variety of preparing methods. U.S. Pat. No. 4,248,803 and U.S. Pat. No. 5,017,542 proposed a catalyst having an improved propylene conversion ratio and offering good acrolein and acrylic acid yields by controlling the composition and content of the catalyst. However, with these methods, it is difficult to keep the suspension of the catalyst for partial oxidation of propylene or isobutylene to acrolein or methacrolein uniform because anionic metal salts and cationic metal salts present in the aqueous solution react with each other and the resultant precipitates sediment quickly. Resultantly, the precipitate is separated from the water layer.
To solve this problem, Korean Laid-Open Patent Publication No. 2002-27023 disclosed a method of preparing a uniform suspension without precipitation of metal using a variety of acids. Although these methods help prepare a uniform suspension, the acids are rapidly decomposed during the calcining process, thereby worsening physical properties and performance of the catalyst.
To summarize, the conventional methods of preparing complex metal oxide catalysts tried to prevent phase separation and reduce suspension particle size in preparing precursor suspensions using an organic acid. While the problems of phase separation and particle size could be solved with the organic acid, the organic acid tends to be decomposed rapidly during the drying or calcining process, so that the surface area and physical properties of the catalyst are negatively affected.